1. Field of Invention
This invention pertains generally to anaerobic digestion and, more particularly, to an anaerobic digestion system and method in which the pH control, mixing, and blending of the biological phases of the anaerobic process in the digestion of biosolid feedstock is accomplished in a single tank or vessel utilizing vertical phase separation, thus eliminating the necessity of separate or additional tanks to provide and/or maintain these functions.
2. Related Art
Anaerobic digestion of biosolids for the reduction of liquid or liquefied wastes such as sewage, garbage, agricultural, and industrial bi-products into energy in the form of methane or natural gas, carbon dioxide, sulfur compounds, and sludge that is useful for fuel or fertilizer, is a multi-phase or stage process. Generally speaking there are two recognized principal phases or types of anaerobic bacteria commonly known as the acid formers and the methane fermenters. In addition there are a number of minor phases or biochemical reactions that take place in the transition between these two major phases, designated herein simply as the transitional phase. There is ample evidence today that some separation of these phases contributes to the digestion efficiency overall, Dague and Kaiser U.S. Pat. No. 5,525,228, and it may be found advantageous to separate any number of these phases, Steiner U.S. Pat. No. 5,630,942.
However, historically and until relatively recent time anaerobic digestion took place in a single tank or vessel. Noted improvement in the anaerobic process has been demonstrated by separating the phases of digestion by using 2 to 4 tanks connected in series. Mixing of the liquor has been on a tank-wise basis in such a manner that the operation of each of the biochemical phases was completely intermixed in a single tank, and phase separation was accomplished by the use of multiple tanks. There are practitioners who believe that the total separation of the major phases is most efficient while other practitioners believe that a certain amount of feedback and blending is necessary in order to maintain the maximum bacterial growth rate and process stability. This process provides the ultimate flexibility to operate in either of these modes as well as any combination of the two. The varying conditions encountered in the anaerobic process during start-up, up-set, recovery, and normal variances in the constitution of the bio-feedstock dictate that the operation of such processes must have the ability to constantly separate, monitor and control the pH, feedback, temperature, and mixing of the liquor in the various phases of the digestion process. The invention described herein provides all of these features in the enclosure of a single tank or vessel.
It is in general an object of the invention to provide an new and improved anaerobic digestion system and method.
Another object of the invention is to provide an anaerobic digestion system and method of the above character which overcomes the limitations and disadvantages of systems and methods heretofore provided.
These and other objects are achieved in accordance with the invention by providing an anaerobic digestion system and method in which the pH control, mixing, and blending of the biological phases of the anaerobic process in the digestion of biosolid feedstock is accomplished in a single tank or vessel utilizing vertical phase separation, thus eliminating the necessity of separate or additional tanks to provide and/or maintain these functions. The invention also provides the ability to control the pH of the liquor in each phase by a hydro mechanical means, while at the same time providing a mixing of the liquor within each phase and allows for feedback and blending of the liquor as may be required for optimum performance.